Disk scheduling basics

[bob]

I see you wondering how the disk arm handles all those requests piling up at once. You notice the head has to chase after data blocks scattered everywhere. That movement eats up precious time and slows everything down. You get why we need smart ways to order those accesses instead of letting them happen randomly. The basic idea is to cut down on how far the arm travels between jobs.
You find that first come first served just lines them up in arrival order. But that often makes the head swing wildly across the platter. I remember testing it and watching seek times balloon with mixed workloads. You can see processes waiting forever if big jumps keep happening. Then shortest seek time first tries to grab the closest request next. It saves on motion but risks starving far away ones indefinitely. You end up with some jobs hanging while others finish quick.
Now scan sweeps the arm in one direction like an elevator picking up passengers. You watch it reverse only at the end after servicing everything that way. Circular scan resets to the start after hitting the edge without going back the same path. That evens out the wait times better for everyone involved. Look versions stop short of the edges if no more requests sit beyond current spots. You gain efficiency because the arm avoids empty travels. Rotational delays also factor in when the platter spins under the head. You combine those with transfer speeds to judge overall throughput.
Perhaps you tweak these for real setups where bursty loads hit the controller hard. I notice fairness suffers in simple shortest first approaches during heavy contention. But combining direction bias with limited look ahead smooths hiccups nicely. You measure success by average response times across varied request patterns. The arm churns less and data flows steadier under tuned methods. Or you factor cylinder skews and zone bit recording into the math for finer gains. Maybe modern drives hide some of this behind firmware smarts yet the basics still apply when you tune at the OS layer.
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